drm/i915: Disable RC6 on Sandybridge by default

RC6 fails again.

> I found my system freeze mostly during starting up X and KDE. Sometimes it
> works for some minutes, sometimes it freezes immediatly. When the freeze
> happens, everything is dead (even the reset button does not work, I need to
> power cycle).

> I disabled RC6, and my system runs wonderfully.

> The system is a Z68 Pro board with Sandybridge i5-2500K processor, 8
> GB of RAM and UEFI firmware.

Reported-by: Kai Krakow <hurikhan77@gmail.com>
Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/gpu/drm/i915/intel_display.c

@@ -7922,13 +7922,11 @@ static bool intel_enable_rc6(struct drm_device *dev)
 		return 0;
 
 	/*
-	 * Enable rc6 on Sandybridge if DMA remapping is disabled
+	 * Disable rc6 on Sandybridge
 	 */
 	if (INTEL_INFO(dev)->gen == 6) {
-		DRM_DEBUG_DRIVER("Sandybridge: intel_iommu_enabled %s -- RC6 %sabled\n",
-				 intel_iommu_enabled ? "true" : "false",
-				 !intel_iommu_enabled ? "en" : "dis");
-		return !intel_iommu_enabled;
+		DRM_DEBUG_DRIVER("Sandybridge: RC6 disabled\n");
+		return 0;
 	}
 	DRM_DEBUG_DRIVER("RC6 enabled\n");
 	return 1;